Membranes useful for the separation of gaseous mixtures are of two very different types: one is microporous while the other is nonporous. Discovery of the basic laws governing the selectivity for gases effusing through a microporous membrane is credited to T. Graham. When the pore size of a microporous membrane is small compared to the mean-free-path of non-condensable gas molecules in the mixture, the permeate is enriched in the gas of the lower molecular weight. Practical and theoretical enrichments achievable by this technique are very small because the molecular weight ratios of most gases are not very large and the concomitant selectivities are proportional to the square roots of these ratios. Therefore, a large number of separation stages is needed to effect an efficient separation of a given gas from a gaseous mixture. However, because this method of separation relies solely on mass ratios and not chemical differences among the effusing species, it is the only membrane based method capable of separating isotopes of a given element. For this reason, this method was chosen to enrich uranium in the fissionable isotope 235 for development of the atomic bomb during World War II. However, this method of separation is inherently expensive due to the large amount of capital investment needed for processing a necessary large amount of gas, stringent membrane specifications requiring high porosity and small pore size, and high energy requirements for operation.
In nonporous membrane systems, molecules permeate through the membrane. During permeation across the nonporous membrane, different molecules are separated due to the differences of their diffusivity and solubility within the membrane matrix. Not only does molecular size influence the transport rate of each species through the matrix but also the chemical nature of both the permeating molecules and the polymer matrix itself. Thus, conceptually useful separations should be attainable.
The art is replete with processes said to fabricate membranes possessing both high selectivity and high fluxes. Without sufficiently high fluxes the required membrane areas required would be so large as to make the technique uneconomical. It is now well known that numerous polymers are much more permeable to polar gases (examples include H2O, CO2, H2S, and SO2) than to nonpolar gases (N2, O2, and CH4), and that gases of small molecular size (He, H2) permeate more readily through polymers than large molecules (CH4, C2H4).
Utilization of membrane separation has taken an important place in chemical technology for use in a broad range application. Gas separation has become a major industrial application of membrane technology in the last 15 years. Membrane based technology for the production of nitrogen from air, removal of carbon dioxide from natural gas, and purification of hydrogen now occupy significant shares of the markets for these processes.
Some of the most difficult separations in the petrochemical industry involve the separation of light olefins and paraffins. Due to their similar relative volatilities, energy-intensive, multi-trayed distillation columns are used for the purification of light olefins. The use membranes has been of interest for many years for the separation of olefins and paraffins. U.S. Pat. Nos. 3,758,603 and 3,864,418 in the names of Robert D. Hughes and Edward F. Steigelmann describe membranes used in conjunction with metal complexing techniques to facilitate the separation of ethylene from ethane and methane. Similar metal complex and membrane hybrid processes, called facilitated transport membranes, have been described in U.S. Pat. No. 4,060,566 in the name of Robert L. Yahnke and in U.S. Pat. No. 4,614,524 in the name of Menahem A. Kraus. Most of this work focused on details of the internals of the facilitated transport membrane device and not on how to incorporate them into a process that produced products that met market specifications.
Processes for the purification of olefins with membranes has focused on the use of facilitated transport membranes in conjunction with distillation columns. A. Sungpet et al. state in an article entitled “Separation of Ethylene from Ethane Using Perfluorosulfonic Acid Ion-Exchange Membranes” published in ACS Symposium Series “Chemical Separations with Liquid Membranes,” 270–285 (1996) that the selectivity and permeability of membranes for the separation of olefins from paraffins is too low to be attractive, so membranes have been combined with other separation processes to achieve the desired separation. We believe that the combination of membranes with distillation is also attractive for another reason: it allows for the maximum use of the vast amount of installed distillation capacity for the purification of olefins.
One of the first studies to examine the combination of facilitated transport membranes with distillation for the separation of olefins and paraffins was published by D. Gottschlich and D. Roberts in a paper for SRI Project 6519 and DOE Contract Number DE-AC07–76ID01570 entitled “Energy Minimization of Separation Process Using Conventional/Membrane Systems” (1990). They examined the application of a facilitated transport membrane to the bottom of a distillation column for the separation of propylene and propane. Since propylene (the olefin) is both the preferentially permeating component and the light component present in low concentration at the bottom of the column, this option appears unattractive because the low driving force leads to very large membrane areas.
Work by R. Noble and co-workers in two articles entitled “Analysis of a Membrane/Distillation Column Hybrid Process” published in J. Memb. Sci. 93, 31–44 (1994) and “Design Methodology for a Membrane/Distillation Column Hybrid Process” published in J. Memb. Sci. 99, 259–272 (1995) examined the design and optimization of several combined facilitated transport membrane and distillation processes for the separation of propylene and propane. Their work focused on the placement of the membrane around the distillation column in order to obtain an efficient process that accomplished the desired separation. They concluded that placing the facilitated transport membrane on the top of the column was preferred since this location takes advantage of the high propylene driving force (due to high propylene concentration).
Earlier work described in U.S. Pat. No. 5,057,641 in the names of Ronald J. Valus et al. and published by J. Davis et al. in an article entitled “Facilitated Transport Membrane Hybrid Systems for Olefin Purification” published in Sep. Sci. Tech 28, 463–476 (1993) also described placing a facilitated transport membrane on the top of a distillation column. This work also described the placement of a facilitated transport membrane on the sidedraw of a distillation column.
The work with silver-based facilitated transport membranes begun by R. Hughes described in U.S. Pat. No. 3,758,603 in 1973 continues today. However, an article recently published by A. Morisato et al. entitled “Transport properties of PA12-PTMO/AgBF4 solid polymer electrolyte membranes for olefin/paraffin separation” in Desalination 145, 347–351 (2002) indicates that the application of facilitated transport membranes continues to encounter difficulties including poor chemical stability due to carrier poisoning.
Advances in polymer membranes make them attractive candidates for olefin/paraffin separations since they do not depend on easily poisoned metal complexes to achieve the separation. For example, R. Burns and W. Koros present several polymeric materials that could be used for the separation of propylene and propane in a recent article entitled “Defining the Challenges for C3H6/C3H8 Separation Using Polymeric Membranes,” J. Memb. Sci. 211, 299–309 (2003).
For polymeric membranes, a large pressure gradient across the membrane would supply the driving force for permeation. This driving force would induce a cooling in the membrane (for materials with positive Joule-Thomson coefficients) in order to produce the low pressure permeate. This affect is not present in facilitated transport membranes and has not been incorporated in previous processes based on them.
Little attention has been given to the heat balance around the membrane apparatus in the general membrane community, primarily because components previously considered for membrane based separations (nitrogen, oxygen, carbon dioxide, methane, hydrogen) are fixed gases. As membrane separations are examined for components that can exist both as a liquid and a vapor at typical industrial process conditions, there is a need to understand the effects of phase transformations on the performance of membrane apparatus.
There is, therefore, a present need for processes and apparatus using perm-selective membranes to provide heat integrated membrane apparatus where pressure-driven (fugacity-driven) membranes have been integrated with other processing steps for the separation of mixtures.
Improved apparatus should provide for an integrated sequence, carried out with streams in gas and/or liquid state, using a suitable perm-selective membrane, preferably a solid perm-selective membrane which under a suitable differential of a driving force exhibits selective permeability of a desired product, i.e., incorporate pressure-driven (fugacity-driven) membranes with existing separation assets.